In a conventional heavy-duty pneumatic tire, bead portions (near rim flanges) are enormously deformed in many occasions. Accordingly, for the purpose of protecting a carcass at each of the bead portions, a cord reinforcing layer using steel cords is disposed to suppress the deformation of the bead portion.
However, the disposing of the cord reinforcing layer for suppressing the deformation increases a shear stress at the bead portions. As a result, heat generation at the bead portions during the running of the tire becomes higher than those at the other portions of the tire, thus bringing about a phenomenon in which rubber is deteriorated by the heat generation.
This deterioration in turn causes a phenomenon in which oxygen in the air filled in the tire permeates into tire rubbers. The permeation of oxygen in the air filled in the tire accelerates, at the bead portions, the deterioration due to the oxidation deterioration of rubber, as compared with the other portions (side portions and the like) of the tire. As a result, a failure, such as occurrence of separation due to reduction in break elongation, is more likely to occur. Moreover, a brake heat is conducted to the bead portions via the rim, thus further accelerating the deterioration of the peripheral portions of the bead portions.
Such deterioration of the bead portions due to oxidation deterioration can be observed as change with time in tire performances, such as the inner pressure holding performance and durability performance of the tire. In particular, in the field of heavy-duty pneumatic tires, tires capable of suppressing such oxidation deterioration have been desired.
In a general pneumatic tire, an inner liner made of rubber excellent in air impermeability is disposed in a surface, on the inner cavity side, of the tire. In this respect, proposed is a tire structure in which an inner liner made of a rubber material, such as a butyl rubber, excellent in the air impermeability is disposed in such a manner as to wrap around a bead toe portion then extending therearound to the outer side, in the axial direction, of the tire (Patent Document 1).
However, in the case of the proposed structure, since the adhesion of a butyl rubber and a diene rubber is normally weak, the bead toe portion is sometimes chipped off when the tire is fitted onto or removed from a rim. By contrast, if a lower end portion of the inner liner is extended only to a position slightly above a distal end of the toe, there is the following disadvantage. Specifically, in this case, a small region of the inner surface side of the tire is not covered by the inner liner. Oxygen and moisture in the air gradually permeate through this region, since the heavy-duty tire has a high inner pressure for use. Eventually, a minute crack occurs in an end of a steel reinforcing cord due to rust or a reduction in adhesion. Such a minute crack grows and thus leads to chafer separation or separation at a carcass turn-up edge.    Patent Document 1: Japanese patent application Kokai publication No. 2001-23013